The Cosmic Microwave Background Radiation

The Cosmic Microwave Background Radiation

Russian-born physicist George Gamov chose to examine what physical implications would result
from an earlier, extremely hot and compact universe [18]. Gamov’s model predicted in 1948 a
radiation pervading all of space that would now be cooled down to a very low temperature
following a lengthy expansion. The explanation for this radiation is the premise that all
fundamental particles would have existed in very close proximity in the early universe, when
extreme temperatures would have caused continued emission and absorption of radiation by the
hot, dense mixture of particles. But as the universe expanded and cooled, one very peculiar
event would make the universe suddenly transparent to this radiation, allowing it to expand and
cool independent of matter in the universe. That event was the point at which the temperature
had fallen sufficiently to allow the slower electrons to be captured by protons, forming neutral
Hydrogen atoms. A similar process forming neutral Helium atoms would occur as well, a topic
we shall return to later. When this happened, the radiation was no longer energetic enough to
free the electrons from the protons, thus allowing no more absorption of the radiation. The
initially extremely hot “explosion” that sent the universe expanding would eventually cool down
as it expanded to a very cold background radiation today. It is called the Cosmic Microwave
Background Radiation or CMBR because it has the same wavelengths as the radiation in the
microwave ovens we use. Not all scientists were impressed by such a clever idea. As a mocking
gesture, Fred Hoyle nicknamed this a “Big Bang” theory, a label that would stick.
This radiation would not look like mere random noise, since it should have a very characteristic
distribution of wavelengths described by “blackbody” radiation. While all objects radiate heat in
the form of light, most objects are not hot enough to radiate light that our eyes can see. For
example, our bodies radiate infrared light, which is invisible to our eyes. In the nighttime sky we
see light coming only from the Moon and planets and stars far beyond our Sun. But if we could
see microwave light, the entire night sky would be glowing. Blackbody radiation is radiated by
an object that absorbs all light incident upon it (rather than reflecting it), then emits the light with
a characteristic distribution of wavelengths that depends only on its temperature. Gamov and his
colleagues had predicted the CMBR temperature to be only around 5 degrees above absolute
zero degrees Kelvin, or -450 °F [19]. At that time it was assumed that this very low temperature
background would be beyond the capability of available technology to detect for many decades.
That brings us to the early 1960’s, when Bell Laboratories scientists Robert Wilson and Arno
Penzias were developing a radio-microwave receiver to study various microwave sources in the
Milky Way Galaxy. The cosmological prediction of Gamov was completely unknown to them.
In fact, Robert Wilson regarded the “Steady-State” theory of Fred Hoyle as the leading
cosmological model [20]. Their interest was more in pinpointing astronomical sources of
microwaves, since it was known that the Milky Way Galaxy is a source of longer radio waves.
They soon detected a faint microwave source that appeared to be coming from all directions in
the sky. This was at first assumed to be noise, either associated with the receiver or with
unwanted background sources nearby. One by one, Wilson and Penzias ruled out the possible
sources of noise. Even the radioactivity in pigeon dung was ruled out after a careful cleaning of
the receiver. The persistence of Wilson and Penzias allowed for an unmistakable identification
of the source of the microwave radiation as an astronomical one by 1965 [21]. But first they had

to make some phone calls to ask for ideas from astronomers. They learned from Princeton
astronomer Jim Peebles that they were looking at the radiation left over from the hot initial
explosion predicted by the Big Bang theory of cosmology. Robert Wilson and Arno Penzias
would be awarded the Nobel Prize in 1978 for their remarkable discovery of the Cosmic
Microwave Background Radiation.
The CMBR has since then been carefully measured by a number of experiments. The Cosmic
Background Explorer or COBE, a satellite launched in 1989, has carefully measured the
radiation to have a temperature of 2.726 degrees Kelvin. However, COBE was designed to
investigate something much more interesting than the precise average temperature of the
radiation. It was recognized that a perfectly uniform temperature from all directions would not
be possible in a universe in which matter had clumped together to form galaxies. The fact that
galaxies did form implied that even during the very early stages of the universe, when the
radiation became transparent to matter, there must have been some unevenness or “ripples” in
the radiation. Without regions with slighter hotter temperatures, there would not have been any
preferred locations for galaxies to begin forming. These ripples were calculated to be minimally
one part in a hundred thousand slightly hotter than the surrounding temperatures. Measuring
such precision low temperatures from earth is rendered very difficult due to the heat of the
earth’s atmosphere. However, COBE could avoid this problem by measuring it in the vacuum of
space far above the earth’s atmosphere. In 1993 COBE scientists announced the discovery of
ripples of a few parts in a hundred thousand [22], showing the radiation has enough unevenness
to account for galaxy formation. This has been confirmed by succeeding experiments. The Big
Bang theory has withstood the test of a closer examination in convincing fashion.